Energy exchange and entropy for quasi-free fermionic semigroups

TL;DR

This paper models energy exchange and entropy in fermionic quantum systems, revealing limitations of thermal machines and analyzing large deviations in energy fluxes through a mathematical framework and numerical simulations.

Contribution

It introduces a model of fermionic semigroups with energy flux analysis and develops a method to compute large deviations via a Riccati equation.

Findings

Thermal machines on quasi-free fermions cannot extract energy from the coolest bath.

Large fluctuations in energy exchanges increase with chain length.

The cumulant generating functional reduces to solving a Riccati equation.

Abstract

We consider a model of quantum dynamical semigroup on a finite dimensional fermionic space, obtained as the continuous-time limit of a repeated interactions model between a system and several thermal baths, with a dynamic driven by quadratic Hamiltonians. We assume that there is a globally conserved observable which can be expressed as a sum of energies on the system and on each baths, and we study the energy fluxes between the baths and the system. First, we consider only the mean energy fluxes, and prove that every thermal machines on quasi-free fermions in trivial, in the sense that it is not possible to extract energy from the coolest bath, even when we dispose of several other baths at different temperatures. Then, we consider an unraveling of the semigroup as a random process, and we study the large deviations of the energy fluxes, following Jaksic, Pillet and Westrich (2014). We reduce the computation of the cumulant generating functional to the resolution of a Riccati equation (which is formally similar to the study of large deviations in classical networks of harmonic oscillators). We apply it to the numerical computation of the rate function for energy exchanges in a fermionic chain, and show that larger fluctuations are observed on a longer chain.